1. Field of the Invention
This invention relates to a process and apparatus for a surface combustor-fluid heater in which combustion is carried out within the pores of a stationary porous bed and heat transfer is achieved using heat exchange surfaces embedded in the stationary porous bed resulting in very high combustion intensity, very high heat transfer rates, improved energy utilization efficiency, ultra-low combustion emissions, and lower capital and operating costs.
2. Description of the Prior Art
In general, heat energy may be transmitted by conduction, convection and/or radiation. Heat transmission by radiation and utilization of infrared energy has many advantages over conventional heat transmission by convection and conduction. The operation and construction of infrared burners and radiant heaters is relatively simple, and thus more economical than other types of heat generation means. The intensity of radiant heat may be precisely controlled for greater efficiency and infrared energy may be focused, reflected, or polarized in accordance with the laws of optics. In addition, radiant heat is not ordinarily effected by air currents. One type of gas-fired infrared generator currently available is a surface combustion infrared burner having a radiating burner surface comprising a porous refractory. The combustion mixture is conveyed through the porous refractory and burns above the surface to heat the surface by conduction. One such burner is taught by U.S. Pat. No. 1,331,022. Other surface combustors are taught by U.S. Pat. Nos. 4,666,400, 4,605,369, 4,354,823, 3,188,366, 4,673,349, 3,833,338, and 4,597,734. See also U.S. Pat. No. 3,738,793 which teaches an illumination burner having a layered porous structure, the layered pores maintaining a stable flame in a thoria-ceria illumination burner in which combustion occurs not within the pores of the combustor, but rather on the surface of the top layer.
Control of combustion emissions, in particular NO.sub.x emissions, is an important requirement for surface combustors which are generally known to produce high combustion intensity and, thus, high combustion temperatures. It is generally known that to reduce NO.sub.x formation within the combustion process, it is necessary to simultaneously remove heat from the combustion process as combustion of the fuel occurs. U.S. Pat. No. 5,014,652 teaches a fluidized bed combustion reactor/fluidized bed cooler comprising a vertical reactor chamber designed to contain two separate fluidized beds, one of which contains cooling coils through which a cooling fluid is flowing to remove heat from the bed. U.S. Pat. No. 3,645,237 teaches a fluidized bed water heater in which water is heated or steam is produced by passing water through heating coils embedded in the fluidized bed. Similarly, U.S. Pat. No. 4,499,944, U.S. Pat. No. 4,779,574, and U.S. Pat. No. 4,646,637 teach a heat exchanger installed in a fluidized bed.
U.S. Pat. No. 4,966,101 teaches a fluidized bed combustion apparatus having a plurality of catalyst tubes filled with catalysts for reforming hydrocarbon gas into steam and arranged in both a horizontal and vertical direction both in and above a fluidized bed in a fluidizing chamber. U.S. Pat. No. 4,899,695 teaches a fluidized bed combustion reaction in which heat is transferred from the fluidized bed to water-containing tubes surrounding the reactor.
U.S. Pat. No. 4,865,122 teaches a fluidized bed heat exchanger for enhanced heat transfer between two liquids having different heat content in which a first liquid is directed through a shell enclosure containing a bed material supported on a distribution plate, the pressure of the liquid controlling the level of fluidization of the bed material, and a second liquid is directed through tubes positioned in the bed material, each of which tube containers includes bed materials supported on a distribution plate. The second liquid is provided at sufficient pressure through the tube containers to fluidize the bed material therein.
U.S. Pat. No. 5,054,436 teaches a recycle bubbling bed formed integrally with a furnace which functions as a heat exchanger and a combustor in which flue gases and entrained particulate materials from a circulating fluidized bed in the furnace are separated, the flue gases are passed to a heat recovery area while the separated solids are passed to the recycle bubbling fluidized bed, and heat exchange surfaces are provided in the recycle bubbling bed to adsorb combustion heat and the solids' sensible heat, and a bypass compartment is provided in another compartment of the recycle bubbling bed through which the solids directly pass to a circulating bed in the furnace during start-up and low load conditions.
U.S. Pat. No. 5,026,269 teaches a nozzle bottom comprising a plurality of fluidizing nozzles for introducing fluidizing air into the reactor chamber of a fluidized bed reactor.
One problem associated with fluidized bed combustors is the amount of particulate matter generated by such beds which is carried out with the products of combustion exhausted by the combustor. In addition, the abrasiveness of the fluidized bed particles against the outer surfaces of heat exchangers disposed in the fluidized bed causes erosion of the heat exchanger surfaces. Finally, pressure drop of flow through the fluidized bed is high due to the high flow velocity required for fluidization.